A grain-oriented electrical steel sheet is a soft magnetic material having very excellent magnetic properties in the rolling direction as a result of the so-called Goss texture, in which all the grains of the steel are oriented in the (110) direction and the crystallographic orientation in the rolling direction is parallel to the [001] axis. Generally, magnetic properties can be expressed as magnetic flux density and core loss, and high magnetic flux density can be obtained by arranging grains in the (100)[001] orientation. An electrical steel sheet having high magnetic flux density can contribute to reducing the size of the core material of an electrical system and also has a low hysteresis loss, and thus can contribute to a decrease in the size and an increase in the efficiency of an electrical device. As used herein, the term “core loss” refers to the loss of power consumed as heat energy when an alternating magnetic field is applied to a steel sheet. The core loss of a steel sheet greatly changes depending on the magnetic flux density and thickness of the steel sheet, the amount of impurities in the steel sheet, and the resistivity and recrystallized grain size of the steel sheet. As the magnetic flux density and resistivity of a steel sheet increase and the thickness and impurity content of the steel sheet decrease, the core loss decreases, resulting in an increase in the efficiency of an electrical device comprising the steel sheet.
Currently, in order to combat global warming worldwide by reducing the generation of CO2, the development of energy-saving and high-efficiency products is ongoing, and as the demand for highly efficient electrical devices that use a reduced amount of electrical energy increases, the demand for grain-oriented electrical steel sheets having low core loss properties increases.
Generally, a grain-oriented electrical steel sheet having excellent magnetic properties should have a strong Goss texture having the (110)[001] orientation, which is formed in the rolling direction of the steel sheet, and in order to form this Goss texture, Goss-oriented grains should grow into abnormal grains which are secondary recrystallized grains. Unlike conventional grain growth, this abnormal grain growth occurs when the migration of grains that normally grow is inhibited by precipitates, inclusions, or elements which form a solid solution or segregate to the grain boundaries. Such precipitates or inclusions that inhibit grain growth are specifically called “grain growth inhibitors,” and studies on the technology of manufacturing grain-oriented electrical steel sheets having the (110)[001] orientation by secondary recrystallization have been focused on ensuring excellent magnetic properties by forming secondary recrystallized grains having high (110)[001] orientation using a strong grain growth inhibitor.
Initially developed grain-oriented electrical steel sheets were manufactured by a two-step cold rolling process using MnS as a grain growth inhibitor as proposed by M. F. Littman. In this process, secondary recrystallized grains were stably formed, but the magnetic flux density of the steel sheet was not sufficiently high and the core loss was undesirably high.
Since then, Taguchi and Sakakura have proposed a method of manufacturing a grain-oriented electrical steel sheet by one-step cold rolling at a cold rolling ratio of 80% or higher using a combination of AlN and MnS precipitates.
Recently, Japanese Patent Publication Nos. Hei 1-230721 and 1-283324 disclose a method for manufacturing a grain-oriented electrical steel sheet, which comprises cold-rolling a steel sheet once without using MnS, decarburizing the cold-rolled steel, and subjecting the decarburized steel sheet to secondary recrystallization using an Al-based nitride exhibiting a strong effect of inhibiting grain growth, in which the secondary recrystallization is caused by introducing nitrogen into the steel sheet in a separate nitrification process using ammonia gas.
Until now, almost all steel manufacturing companies that manufacture grain-oriented electrical steel sheets have used a manufacturing method in which precipitates such as AlN or MnS[Se] are mainly used as grain growth inhibitors to cause secondary recrystallization.
This method for manufacturing a grain-oriented electrical steel sheet using an AlN or MnS precipitate as a grain growth inhibitor has an advantage in that stable secondary recrystallization can be induced, but the precipitates should be very finely and uniformly distributed in the steel sheet to exhibit a strong effect of inhibiting grain growth. In order to uniformly distribute the fine precipitates as described above, a slab should be heated to a temperature of 1,300° C. or higher for a prolonged period of time before hot rolling so that coarse precipitates present in the steel are dissolved to form a solid solution, and then within a very short time, the steel should be hot-rolled before precipitation occurs. For this purpose, a large-scale slab heating system is required, and for the maximal inhibition of precipitation, hot rolling and coiling processes should be very strictly managed, and a process of annealing the hot-rolled sheet should be managed such that the solid solution is finely precipitated. In addition, when the slab is heated to a high temperature, Fe2SiO4 having a low melting point will be formed, resulting in slab washing, thus reducing the yield of the steel sheet.
In addition to the above-described problems, in the grain-oriented electrical steel sheet manufacturing method in which secondary recrystallization is caused using an AlN or MnS precipitate as a grain growth inhibitor, purification annealing should be carried out at a temperature of 1,200° C. or higher for hours or longer after completion of secondary recrystallization, in order to remove precipitates. This purification annealing complicates the manufacturing process and increases the manufacturing cost.
In other words, when precipitates such as AlN or MnS remain in the steel sheet after causing secondary recrystallization using the precipitates as grain growth inhibitors, they interfere with the movement of magnetic domains to increase the hysteresis loss. For this reason, the precipitates should necessarily be removed. Thus, after completion of secondary recrystallization, refinement annealing is carried out using 100% hydrogen gas at a high temperature of about 1,200° C. for a prolonged period of time to remove precipitates such as AlN and MnS, as well as other impurities. In this purification annealing, the MnS precipitate is separated into Mn and S, and the separated Mn is dissolved in the steel, and the separated S diffuses to the surface of the steel and reacts with atmospheric hydrogen gas to form H2S, which is discharged.
In recently developed technology for manufacturing a grain-oriented electrical steel sheet using a low-temperature slab heating method that forms secondary recrystallized grains by AlN-based precipitates resulting from nitrification after decarburization annealing following cold rolling, a slab is heated at a temperature of 1,200° C. or lower in order to overcome problems such as difficulty in the operation of a slab heating system and a decrease in yield in the hot rolling step. However, in this method, purification annealing should also be carried out at a high temperature of 1,200° C. or higher for 30 hours or longer after completion of secondary recrystallization in order to remove the components of the AlN precipitates, and this purification annealing complicates the manufacturing process and increases the manufacturing cost.
In this purification annealing process, the AlN-based precipitate is separated into Al and N, after which the separated Al migrates to the surface of the steel sheet and reacts with oxygen on the surface to form an Al2O3 oxide. The formed Al-based oxide or the AlN precipitates which are not separated in the purification annealing process interfere with the movement of magnetic domains in the steel sheet or portions close to the steel sheet surface to increase the core loss.
Thus, in order to improve the magnetic properties of a grain-oriented electrical steel sheet and reduce the dependence of the steel sheet on purification annealing to increase the steel sheet productivity, new technology for manufacturing a grain-oriented electrical steel sheet, which does not use precipitates such as AlN or MnS as grain growth inhibitors, is required.
Methods for manufacturing a grain-oriented electrical steel sheet without using an AlN or MnS precipitate as a grain growth inhibitor include a method of preferentially growing grains in the (110)[001] orientation using surface energy as the grain growth driving force as disclosed in Japanese Patent Laid-Open Publication Nos. Sho 64-55339 and Hei 2-57635. This method is based on the finding that grains present on the steel sheet surface have surface energy that is different between crystallographic orientations and that (110)-oriented grains having the lowest surface energy grow while encroaching other grains having higher surface energy. This method has a problem in that the thickness of the steel sheet should be thin so that this difference in surface energy is effectively used. Japanese Patent Laid-Open Publication No. Sho 64-55339 discloses a steel sheet thickness of 0.2 mm or smaller, and Japanese Patent Laid-Open Publication No. Hei 2-57635 discloses a steel sheet thickness of 0.15 mm or smaller. In the methods disclosed therein, a grain-oriented electrical steel sheet having excellent magnetic properties can be manufactured only under the condition in which the steel sheet thickness is very small as described above. However, the thickness of grain-oriented electrical steel sheets which are widely used to manufacture electrical transformers is 0.23 mm or larger, and at a product thickness larger than 0.23 mm, there is technical difficulty in forming secondary recrystallized grains using surface energy. Further, the technology employing surface energy has a problem in that a high load acts on a hot rolling process when a steel sheet has a thickness of 0.20 mm or smaller is manufactured. In addition, in order to effectively use surface energy, secondary recrystallization should be performed in a state in which the production of oxide on the steel sheet surface is inhibited as much as possible. For this reason, a high-temperature annealing atmosphere should necessarily be a mixed gas atmosphere of inert gas and hydrogen gas. In addition, because no oxide layer is formed on the steel sheet surface, it is impossible to form an Mg2SiO4 (forsterite) layer in a high-temperature annealing process for forming secondary recrystallized grains, and thus insulation of the steel sheet is difficult and the core loss increases.
Meanwhile, Japanese Patent Laid-Open Publication No. 2000-129356 discloses a method for manufacturing a grain-oriented electrical steel sheet, in which secondary recrystallized grains are formed by minimizing the content of impurities in the steel sheet without the use of precipitates to maximize the difference in the grain boundary mobility of grains between crystallographic orientations. This patent document suggests that the Al content is inhibited to 100 ppm or less and the contents of B, V, Nb, Se, S, P and N are inhibited to 50 ppm or less, but the examples of the patent document describe that a small amount of Al forms precipitates or inclusions to stabilize secondary recrystallized grains. Thus, the method disclosed in the above patent document does not appear to be a method for manufacturing a grain-oriented electrical steel sheet, which completely excludes the use of precipitates, and the magnetic properties of the steel sheet manufactured by the method are inferior to those used to manufacture commercially available, grain-oriented electrical steel sheets. In addition, even though the low core loss properties of the steel sheet are ensured by minimizing the content of impurities in the steel sheet, problems such as low productivity and increased manufacturing costs are not solved.
In addition, there was an attempt to use various precipitates such as TiN, VN, NbN or BN as grain growth inhibitors, but stable secondary recrystallized grains could not be formed due to thermal instability and the excessively high decomposition temperature of precipitates.